I. Field of the Invention
This invention relates to improvements in the structure and method of manufacturing thrust bearings. More particularly, this invention relates to diamond thrust bearings for use in downhole drilling operations and a process for manufacturing same.
II. Background Art
Exploratory bore holes are drilled in the earth to gain access to materials located therein. The cost of exploratory equipment is high, and therefore, many companies prefer to lease exploratory equipment for short-term or sporadic needs. Leased drilling equipment requires large quantities of drilling fluid, skilled manpower, and frequent maintenance. These factors of overhead, when added to the cost of leasing drilling equipment, make even leasing exploratory equipment an expensive process. Downtime from equipment failure or maintenance impinges on the time this expensive equipment is in productive use.
A. The Exploratory Environment
To illustrate the environment in which this exploration takes place, FIG. 1 shows a drill rig 10 erected over a surface 12 of the earth at an area to be explored for oil. A bore hole is drilled below drill rig 10 through surface 12 and strata 13, to a desired depth. Advancement of drilling end 14 into the earth is accomplished by a drill bit 15 which progresses through strata 13 by the action of rotating teeth located on the end thereof.
B. The Downhole Drilling Motor
Drill bit 15 is powered by a downhole drilling motor 16. Downhole drilling motor 16 is located at the end of a series of pipe sections comprising drill string 18. The housing of downhole drilling motor 16 is located in drilling end 14 of drill string 18 and remains stationary with drill string 18 as it powers drill bit 15. Downhole drilling motor 16 is powered by drilling fluid, commonly referred to as drilling mud, which is pumped under pressure through drill string 18 and through downhole drilling motor 16.
Downhole drilling motors such as downhole drilling motor 16 are cylindrical so as to be capable of passing through the bore hole drilled by drill bit 15. Downhole drilling motors must, therefore, conform to the size restriction imposed by the outside diameter of the drill pipe in drill string 18. The length of downhole motor 16, however, often ranges up to thirty feet. Downhole drilling motors utilize the effect of drilling mud pressure and momentum change of drilling mud as it passes through turbine blades to provide torque to turn drill bit 15. Drill bit 15 penetrates earth and rock by a combination of the downward pressure exerted by the weight of drill string 18 and the rotary action imparted to drill bit 15 by downhole drilling motor 16. As the bore hole deepens, additional sections of pipe are added to drill string 18 at drill rig 10.
C. Downhole Thrust Bearings
FIG. 2 illustrates in more detail drilling end 14 of drill string 18 shown in FIG. 1. A casing 17 of downhole drilling motor 16 is shown attached to the last segment of drill string 18. Located within casing 17 of downhole drilling motor 16 are two thrust bearing assemblies, an upper thrust bearing assembly 20 and a lower thrust bearing assembly 21. Downhole drilling motor 16 powers drill bit 15 located at the free end of drilling end 14.
Thrust bearing assemblies 20, 21 allow for the rotation of drill bit 15 relative to casing 17 of downhole drilling motor 16. To maintain the rotation of drill bit 15 when downhole drilling motor 16 is powering drill bit 15, thrust bearing assemblies 20, 21 must be capable of operating under compressive pressure from the weight of drill string 18 and tensile pressure from the force of the pressurized drilling mud passing through downhole drilling motor 16.
1. Off-bottom Thrust
Drilling mud is pumped through drill string 18 to downhole motor 16 in a direction shown by arrow A, a direction referred to as "downhole." The high pressure drilling mud exerts a force downhole on drilling motor 16 that tends to push drilling motor 16 toward the bottom 22 of bore hole 23. This force is referred to as "off-bottom thrust" since the thrust is exerted whenever drilling mud is pumped through downhole drilling motor 16 and drill bit 15 is off the bottom of bore hole 23.
2. On-bottom Thrust
When drill bit 15 is in contact with bottom 22 of bore hole 23, the weight of drill string 18 exerts a force on drilling end 14 which tends to compress drilling motor 16. This force is referred to as "on-bottom thrust," as it is experienced only when drill bit 15 is in contact with bottom 22 of borehole 23.
During drilling, on-bottom thrust caused by the weight of drillstring 18 is countered by off-bottom thrust caused by the hydraulic pressure of the drilling mud. The interaction of on-bottom and off-bottom thrusts lessens the overall thrust that must be borne by thrust bearing assemblies 20, 21, during actual drilling.
Periodically, during the drilling process, drill bit 15 wears out requiring drill string 18 to be pulled up out of bore hole 23 to gain access to drill bit 15. After replacing drill bit 15, drill string 18 is reassembled as drill bit 15 is lowered back into bore hole 23. During this period of lowering drill string 18 back to the previously achieved depth, drilling mud is pumped under pressure through drill string 18 to turn downhole drilling motor 16 and thereby cause drill bit 15 to rotate and clean bore hole 23 as drill bit 15 descends.
The period during which drill bit 15 is descending into bore hole 23 exposes thrust bearing assemblies 20, 21 to off-bottom thrusts caused by drilling mud pressing downhole drilling motor 16 in a downhole direction. Thrust bearing assemblies 20, 21, do not have the advantage of offsetting on-bottom thrust during this time, and so, must bear the entirety of the off-bottom thrust. Typical on-bottom thrusts may exceed 40,000 pounds and off-bottom thrusts may exceed 30,000 pounds.
3. Diamond Drill Bits
Previously, a typical drill bit would last approximately fifteen hours before needing replacement. To lengthen the interval between drill bit replacement, drill bits were introduced which incorporated synthetic diamonds into the surface of the drill bit. These diamond drill bits have increased the useful life of drill bit 15 from fifteen hours to one hundred fifty hours. This increase in useful life allows much longer intervals before drill-bit replacement is necessary.
D. The Evolution of the Diamond Thrust Bearing
With the introduction of diamond drill bits, however, a new problem arose. While the new diamond drill bits had a useful life of one hundred fifty hours, the thrust bearings had a useful life of only fifty hours. Thrust bearing assemblies 20, 21 became the limiting factor in downhole operations. When a thrust bearing assembly 20, 21 wears out, drill string 18 must be pulled out of bore hole 23 to access downhole drilling motor 16 and the thrust bearing assemblies 20, 21 contained therein. Thrust bearing assembly failure required drilling to be halted every fifty hours to replace the thrust bearing assemblies 20, 21 in downhole drilling motor 16.
1. Roller Thrust Bearings
To cope with the forces operating on downhole drilling motor 16, the earliest thrust bearings utilized ball bearings travelling in races. Thrust bearing assemblies, such as 20, 21 were positioned at both ends of downhole drilling motor 16 to cope with both on-bottom and off-bottom thrusts. In a first attempt to increase thrust bearing life, ball bearings were replaced with roller bearings to increase the bearing surface carrying the load from on-bottom and off-bottom thrusts.
Roller thrust bearings first used in downhole motors had a useful life of approximately fifty hours. Since drill bits used at the introduction of such bearings had a useful life of only fifteen hours, roller thrust bearings were not a limiting factor in causing downtime. Roller thrust bearings were simply replaced concurrently with drill bit 15 after several intervening drill bit changes. With the introduction of diamond drill bits 15, however, roller thrust bearings became a limiting factor in the efficient use of drilling equipment. The solution to this disparity in useful life between diamond drill bit 15 and roller thrust bearings was to develop thrust bearings with longer useful lifetimes. This was accomplished by incorporating synthetic diamonds into the bearing surfaces of thrust bearings 20, 21.
2. Diamond Thrust Bearings
Diamond thrust bearings are paired to create thrust bearing assemblies 20, 21 like those shown in FIG. 3. Each diamond thrust bearing is manufactured with diamond bearing pad retainer 28 having interference fitted within bearing pad recesses a plurality of diamond bearing pads like bearing pad 26. Diamond bearing pad 26 is cylindrical and comprises a bearing end 32 terminating in a substantially planar bearing face 34. Opposite bearing end 32 is an insertion end 36 which is held in bearing pad retainer 28. Insertion end 36 has a bevel 37 to facilitate insertion into the bearing pad retainer 28. Diamond bearing pad 26 is often constructed of tungsten carbide in which the synthetic diamonds are bonded. The synthetic diamonds of substantially planar bearing face 34 give diamond thrust bearings a useful life that approximates that of diamond drill bits like drill bit 15, substantially increasing the productive operational time that drilling equipment is in use in a given period.
As illustrated in FIG. 4, diamond bearing pads 26 are typically arranged in a circle inside of an annular bearing pad retainer 28. Bearing end 32 projects above a receiving surface 40 and terminates in substantially planar bearing face 34.
Thrust bearing assembly 21 comprises two thrust bearings 29 like that illustrated in FIG. 4. Two thrust bearings 29 are located such that the substantially planar bearing faces 34 of diamond bearing pads 26 of one bearing pad retainer are in contact with the corresponding substantially planar bearing faces 34 of the opposing bearing pad retainer. This orientation assures uniform contact between all diamond bearing faces 34 contributing to the longer useful life of diamond thrust bearing assemblies.
3. The Predetermined Common Bearing Plane
Maximizing the load-carrying capacity of two opposing thrust bearings 29 requires that the load carried by bearing pad retainer 28 be spread over the maximum bearing surface of all of diamond bearing pads 26. To accomplish this, the substantially planar bearing faces 34 of each diamond bearing pad 26 must be parallel with the bearing faces 34 of diamond bearing pads 26 of the opposing bearing pad retainer.
Maximizing the total bearing surface of the overall thrust bearing 29, requires that all bearing faces 34 in each thrust bearing 29 be disposed and must remain disposed coplanar with each other in a theoretical predetermined common bearing plane. Any deviation of a bearing face out of the predetermined common bearing plane contributes to premature thrust bearing failure, as some diamond bearing pads 26 are required to carry a greater load than the others in the bearing pad retainer 28.
4. Bearing Pad Recess Depth
To produce a thrust bearing 29, bearing pad recesses 42 are drilled to precise depths in bearing pad retainer 28. This method requires equipment capable of drilling bearing pad recesses with precise tolerances on a continual basis. This equipment requires frequent adjustment, as the drill bit wears from drilling the bearing pad recesses in the hard bearing pad retainer. This need for constant adjustment results in bearing pad recesses 42 that vary slightly in their dimensions.
As diamond bearing pads 26 are inserted into bearing pad recesses 42, any deviation in bearing pad recess depth results in substantially planar bearing faces 34 that are no longer coplanar with the predetermined common bearing plane.
5. Brazing
To retain diamond bearing pads 26 in bearing pad recesses 42 during drilling operations, diamond bearing pads 26 are brazed into bearing pad recesses 42. Flux is placed in the bottom of each of the bearing pad recesses 42 followed by a diamond bearing pad 26. Bearing pad retainer 28 is then heated to a temperature high enough to braze diamond bearing pad 26 to bearing pad recesses 42. Bearing pad retainer 28 is then cooled to ambient temperature.
As bearing pad retainer 28 cools, however, heat distortion of bearing pad retainer 28 may occur. This heat distortion may cause misalignment of substantially planar bearing faces 34 of diamond bearing pads 26. Heat-distorted bearing pad retainers elevate some bearing faces 34 out of the predetermined common bearing plane, thereby causing some bearing faces 34 to carry more thrust loading than others.
Heat distortion of even a very small degree can result in premature thrust bearing failure. Small distortions caused by heating bearing pad retainer 28 during the brazing process are difficult to discover and are, therefore, difficult to eliminate. Processes designed to establish substantially planar bearing faces 34 coplanar with the predetermined bearing plane during brazing are ineffective in maintaining substantially planar bearing faces 34 in a coplanar orientation during cooling.